Design and in vitro characterization of highly sst2-selective somatostatin antagonists suitable for radiotargeting

Radiolabeled sst(2) and sst(3) antagonists are better candidates for tumor targeting than agonists with comparable binding characteristics (Ginj, M.; Zhang, H.; Waser, B.; Cescato, R.; Wild, D.; Erchegyi, J.; Rivier, J.; Macke, H. R.; Reubi, J. C. Proc. Natl. Acad. Sci. U.S.A. 2006,103, 16436-16441.). Because most of the neuroendocrine tumors express sst(2), we used the known antagonists acetyl-pNO(2)Phe(2)-C[DCys(3)-Tyr(7)-DTrp(8)-Lys(9)-Thr(10)-Cys(14)]-DTyr(15)-NH2 (1) (Bass, R. T.; Buckwalter, B. L.; Patel, B. P.; Pausch, M. H.; Price, L. A.; Strnad, J.; Hadcock, J. R. Mol. Pharmacol. 1996, 50, 709-715. Bass, R. T.; Buckwalter, B. L.; Patel, B. P.; Pausch, M. H.; Price, L. A.; Strnad, J.; Hadcock, J. R. Mol. Pharmacol. 1997, 51, 170; Erratum.) and H-Cpa(2)-c[DCys(3)-Tyr(7)-DTrp(8)-Lys(9)-Thr(10)-Cys(14)] -2Nal(15)-NH2 (7) (Hocart, S. J.; Jain, R.; Murphy, W. A.; Taylor, J. E.; Coy, D. H. J. Med. Chem. 1999, 42, 1863-1871.) as leads for analogues with increased sst(2) binding affinity and selectivity. Among the 32 analogues reported here, DOTA-pNO(2)Phe(2)-c[DCys(3)-Tyr(7)-DAph(8)(Cbm)-Lys(9)-Thr(10)-Cys(14)-DTyr(15)-NH2 (3) and DOTA-Cpa(2)-C[DCys(3)-Aph(7)(Hor)-DAph(8)(Cbm)-Lys(9)-Thr(10)-Cys(14)] -DTyr(15)-NH2 (31) had the highest sst(2) binding affinity and selectivity. All of the analogues tested kept their sst(2) antagonistic properties (i.e., did not affect calcium release in vitro and competitively antagonized the agonistic effect of [Tyr(3)] octreotide). Moreover, in an immunofluorescence-based internalization assay, the new analogues prevented sst(2) internalization induced by the sst(2) agonist [Tyr(3)] octreotide without being active by themselves. In conclusion, several analogues (in particular 3, 31, and 32) have outstanding sst(2) binding and functional antagonistic properties and, because of their DOTA moiety, are excellent candidates for in vivo targeting of sst(2)-expressing cancers.